In recent years, lithium secondary batteries including positive and negative electrodes formed using a material that allows insertion and extraction of lithium ions, which were developed, aimed at on-vehicle applications. It has been a great challenge to implement a lithium secondary battery that achieves high energy density, high output (large-current charge/discharge), long lifetime, and high safety.
Various solutions such as (1) improvements in positive/negative electrode material (Japanese Patent No. 3867030, Sei KK); (2) improvements in the collector foil (WO2011/049153, SEI Corporation); and (3) improvements in the separator (PCT/JP2012/056998) have been proposed to implement such a lithium secondary battery. The content of each of these three documents is incorporated herein by reference in its entirety for all purposes.
Energy density and output of a lithium secondary battery were improved, for example, by reducing the particle size of positive/negative electrode active material particles, increasing the specific surface area of positive/negative electrode active material particles via surface modification or the like, or increasing the electrode area by improving the electrode design. Although these measures have opened the door to a possibility to implement a lithium secondary battery aimed at on-vehicle applications, the improvement in energy density, safety, and lifetime is currently insufficient.
Extensive research and development have been conducted in order to achieve higher energy density. For example, an increase in charge voltage of an Ni-rich LNMC (Li(Ni/Mn/Co)O2) positive electrode material, use of a sulfur compound having high theoretical energy density as a positive electrode material, and use of an alloy-based negative electrode material (or an oxide thereof) having semiconductor properties have been proposed. Lithium-air batteries have also been proposed as novel lithium batteries.
A Li(Ni/Mn/Co)O2/LiFePO4 mixed battery has also been proposed in the Abstracts of 53rd Battery Symposium in Japan (p. 40, November 2012), Committee of Battery Technology, Electrochemical Society of Japan, incorporated herein by reference in its entirety.
The initial energy density of a lithium battery can temporarily be increased by the above means. However, it is difficult to implement the cycle life of 5,000 to 10,000 cycles (10 years) required for vehicle applications which necessitate maintaining a high energy density.
A Ni-rich LNMC positive electrode material achieves long constant-current discharge, but does not exhibit flat voltage characteristics (i.e., generally exhibits voltage characteristics that decrease from the high-voltage region). An on-vehicle battery should exhibit flat voltage characteristics from the viewpoint of quality, high output and high energy density, which characteristics cannot be practically achieved when using a Ni-rich LNMC positive electrode material. More specifically, since an on-vehicle battery is used at a constant power, it is impossible to use an on-vehicle battery up to a considerable discharge depth.
A mixed battery can initially prevent a decrease in output due to the mixed potential, but shows a decrease in output as the number of charge-discharge cycles increases since the reaction site is concentrated on an active material having low resistance.
A lithium battery with improved properties, while avoiding the drawbacks of the preceding examples, for an on-vehicle battery is thus desirable.